Conventionally, as a distance measuring device by the use of electric wave, a pulse radar, an FMCW radar, and other radars are known.
The pulse radar transmits a pulse signal, measures the time in which the pulse signal is reflected by a measurement object, and finds the distance to the measurement object. In addition, the FMCW radar transmits a frequency-swept continuous wave and measures distance to a measurement object from the frequency difference between the transmitted signals and reflected signals. In addition to this, there are spread spectrum radar, coded pulse radar, and others, but these radars measure distance on the basis of round-trip time of signals to a measurement object as is the case of pulse radar.
At the same time, the foregoing radars basically measure round-trip time of signals to a measurement object, and resolution within several tens of meters lacks; therefore, it is difficult to measure the close range within several tens of meters. Furthermore, because the FMCW radar measures distance to the measurement object from the frequency difference between transmitted signals and reflected signals, it has problems of a need of the linearity of frequency change of transmitted signals as well as of a “False Object” which is caused by transmitted signals that leak to the receiving side; it also need accurately output transmitted signals so as to satisfy the linearity of frequency changes, separate antennas on the transmitting and receiving sides so that transmitted signals do not leak into the receiving side to prevent an occurrence of “False Object”, and others, and it has been difficult to achieve a simple construction.
Therefore, there is a distance measuring device, as shown in FIG. 23, which bases on a concept in that a standing wave is generated if there is reflection (reflected wave) from a measurement object when an electromagnetic wave which has only one frequency component is transmitted to the measurement object as a progressive wave from an electromagnetic wave generating source; the device transmits an electromagnetic wave which has only one frequency component to a measurement object while changing over the frequency stepwise, detects a standing wave generated by interference between this transmitted wave and a reflected wave reflected by the measurement object, and finds distance between the detection point and the measurement object on the basis of the result of computing a variable period of amplitudes of this standing wave (see Patent Document 1).
The distance measuring device set forth in patent document 1 detects a standing wave generated by an interference between a progressive wave which has only one frequency component and a reflected wave produced by the progressive wave that reflects against the measurement object; therefore, the distance measuring device does not need to have a transmitted signal prevented from leaking into the receiving side as is the case of the FMCW radar, etc., and a simple construction can be achieved. Furthermore, as compared to a pulse radar, FMCW radar, and other radars, the distance measuring device set forth in patent document 1 is able to measure accurately even in near distance measurement.
The distance measuring device set forth in patent document 1, however, is effective when travel speed between the measurement object and the travel distance measuring device is comparatively slow and the travel distance within the measurement time can be ignored but when the travel speed between the measurement object and the distance measuring device is fast and the travel distance within the measurement time is unable to be ignored, the distance measuring device is difficult to obtain correct measured value due to the Doppler effect.
For measuring a distance accurately to a moving measurement object, there are distance measuring devices which increase and decrease the frequency of a signal which has only one frequency component in accordance with predetermined step frequencies, transmit this as a progressive wave, detect the amplitude of a standing wave generated by the interference between this progressive wave and a reflected wave produced by the progressive wave that reflects against a measurement object, compute signals which correspond to the detected amplitude, and find the distance between the detection point and the measurement object (for example, see patent document 2 and non-patent document 1).
The distance measuring devices set forth in patent document 2 and non-patent document 1 can compute signals which correspond to the amplitude of a standing wave generated by the interference between a progressive wave whose frequency is increased and decreased in accordance with predetermined step frequencies and the reflected wave, and simultaneously measure the distance between the detection point and the measurement object as well as the relative speed of the measurement object. In addition, as is the case of the distance measuring device set forth in patent document 1, the distance between the detection point and the measurement object is found by the use of the standing wave, and a simple construction is able to be achieved as a distance measuring device.
Patent document 1: Japanese Unexamined Patent Application Publication No. 2002-357656
Patent document 2: Japanese Unexamined Patent Application Publication No. 2004-325085
Non-patent document 1: “Short-Range High-Resolution Radar Utilizing Standing Wave for Measuring of Distance and Velocity of a Moving Target,” FUJIMORI Shingo, UEBO Tetsuji, and IRITANI Tadamitsu, IEICE Transactions, vol. J87-B, No. 3, pp. 437-445, March 2004.